It's just an inline cap , it'll be a small value and you can use an online calculator to determine that value.... you just enter the desired -3 db point and the amps input inpedence, it'll give you a vaulue.
Use a very high quality cap for these (( you need one on each channel.
Depending on how Gary has your pre destup, you could also just filter 2 of the ouitputs so they high pass the signal to the Folsom... or, add the caps to the Folsom inouts.
Going either of those 2 ways would eliminate the need for more connectors/cables going to and from an inline filter.
I've run my Dodd pre and the Folsom 7297 , it's a great combo. In fact, at some point I need to dig that gear ouit and give it a go with the Super 7's as I've never run the combo on them.
Have a look here:
https://electronicbase.net/high-pass-filter-calculator/jay